The invention concerns a method for nuclear magnetic resonance (NMR) spectroscopy of a sample comprising the following steps:    (a) excitation of long lived coherences (LLC) between the singlet state S0 and the central triplet state T0 of nuclei of the sample by initiating irradiation of the sample with an rf-field with a carrier frequency;    (b) sustaining of the LLC by maintaining the rf-irradiation during an interval τ2;    (c) converting the LLC temporarily into observable magnetisation by interrupting the rf-irradiation during an observation interval τ3;    (d) detecting NMR-signals during the observation interval τ3.
Most nuclear magnetic resonance (NMR) methods employ Fourier transformations of free induction decays (FID's).1 Though widely used, this approach suffers from homogeneous decay and imperfect homogeneity of a static magnetic field, so that it is challenging to achieve line-widths below 1 Hz.2 Sophisticated NMR pulse sequences have been developed to achieve reasonable line-widths (1<Δν<50 Hz) in moderately inhomogeneous fields, exploiting cross relaxation effects3, observation in the earth's magnetic field4, or a spatial correlation between the static and radio-frequency (rf) field profiles5. By combining refocusing and coherence transfer through couplings, one can obtain acceptable line-widths (1<Δν<50 Hz) even in very inhomogeneous fields (Δν>2 kHz).6 In systems with two scalar-coupled homonuclear spins I=½ and S=½, one can excite long-lived coherences (LLC's) that can have very long life-times TLLC and hence very narrow line-widths ΔνLLC=1/(π/TLLC).7-9 Their precession frequency is independent of offset (and hence of chemical shifts and inhomogeneous broadening) and is only determined by the sum of scalar and residual dipolar couplings (TIS=JIS+2DIS). So far, LLC's have only been observed indirectly in the manner of two-dimensional (2D) spectroscopy, i.e. point by point, either in combination with field cycling7 or in high field.8-9 
Principles
Long-lived coherences (LLC's) constitute a class of zero-quantum coherences that can be excited by extremely low frequency fields (ELF's) in a vanishing static field.7 LLC's can also be excited in high fields by creating a state where the coherences Iy and −Sy have opposite phases, so that they can be locked by a continuous ‘sustaining’ rf field8-9. This rf field in effect suppresses the chemical shifts, thus rendering the spins magnetically equivalent, so that their eigenstates can be classified according to ‘symmetrical’ and ‘antisymmetrical’ irreducible representations of the spin permutation group. LLC's span zero-quantum transitions between states of different symmetry. Their oscillatory decays can be subjected to a Fourier transformation, yielding doublets that are reminiscent of ‘J-spectroscopy’11-13. The life-times TLLC of LLC's can be a factor κ longer than the transverse relaxation times T2=TSQC of ordinary single-quantum coherences (TLLC=κT2), so that the line-widths ΔνLLC=1/(πTLLC) can be narrower by a factor ΔνLLC/ΔνSQC=1/κ. Depending on the role of extraneous relaxation mechanisms9, one can expect κ≦3 in small molecules in the extreme narrowing limit, and κ≦9 in the slow14 motion limit typical of large molecules. In practice, we have observed 2.5<κ<4.3 over a range of correlation times.15 
Generally speaking, LLC's should not be confused with long-lived states (LLS's), also known as singlet states (SS) if there are only two spins in the system. LLS's refer to populations of antisymmetric singlet states16-28. LLS's have life-times that can be much longer than LLC's (TLLS>>TLLC), but do not have any oscillatory character, and cannot give rise to J-spectra in the manner of LLC's. Both LLS's and LLC's can be temporarily converted into observable magnetisation (vide infra). This is of particular interest when the initial polarization is enhanced by ‘dissolution’ DNP10,21.
If the oscillatory decays of LLC's are observed point-by-point in the manner of two-dimensional (2D) spectroscopy, they cannot be enhanced (‘hyperpolarized’) by ‘dissolution’ DNP. Recently, several 2D experiments have been successfully converted into ‘ultra-fast’ versions that can be combined with ‘dissolution’ DNP.22-23 However, the continuous rf field, which is preferably used to sustain LLC's is not compatible with current ‘ultra-fast’ schemes.
It is an object of the present invention to propose a method that allows obtaining ultra high-resolution spectra of long-lived coherences with enhanced resolution.